1. Field of the Invention
The present invention is related to the fabrication method of the bipolar transistor and is especially related to the fabrication method of the self-aligned NPN transistor and the self-aligned lateral PNP transistor having a field plate.
2. Description of the Prior Art
In the fabrication of a lateral PNP transistor, the epitaxial layer serves as the base region.
During the process, the surface of this epitaxial layer is likely to be weakly inverted of p-type or to be depleted, by the autodoping of boron ions and other contaminations.
This problem is severe especially in case that the resistivity of the epitaxial layer is high.
This surface inversion induces the collector-emitter leakage current of the lateral PNP transistor, so the device performance is degraded.
To prevent this surface inversion, the metal field plate overlapped on the epitaxial base region has been used conventionally.
In that case, the emitter metal overlaps over the base region and serves the field plate.
FIG. 1 is the sectional view of the conventional lateral PNP transistor with a metal field plate.
As shown in FIG. 1, p-type emitter 2 and p-type collector 3 are formed on n-type epitaxial layer 1 which serves as the base of lateral PNP.
The emitter contact metal 6 overlaps the base region 7 of lateral PNP transistor to form a metal field plate.
This metal plate prevents the surface inversion in base region of lateral PNP transistor, and reduces the collector-emitter leakage current because the emitter is biased higher than the base, collector in normal operation.
However, this structure has some handicaps.
The device area is increased because of a metal spacing.
Also, the emitter bias voltage has to be high for prevention of the surface inversion because the oxide thickness under the metal field plate is thick.